Gold-spotted oak borers have killed more than 80,000 oak trees in San Diego County over the past decade -- nearly four times as many as scientists figured at the start of 2011.

Researchers at the University of California Riverside released the latest numbers Friday as they stepped up their call for residents and businesses not to move oak firewood from infested areas. They said the new estimate is based on more refined mapping techniques, not an explosion of tree mortality in the past eight months.

Still, the figures provided a sobering reminder about the bug's destructive power and the stakes for containing it. It's only been found in San Diego County so far, a hopeful sign as researchers search for control methods.

Gold-spotted oak borer — Tom Coleman, USDA

“This may be the biggest oak mortality event since the Pleistocene" 12,000 years ago, said Tom Scott, a natural resource specialist for the University of California. “If we can keep firewood from moving out of the region, we may be able to stop one of the biggest invasive pests to reach California in a long time.”

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Field studies in San Diego County over the past six months strongly suggest that oak-killing beetles are moving around mostly through infested firewood. Scott said the beetles don't appear to be roaming quickly on their own and that human movement of oak wood is the best explanation for oak mortality patterns in the backcountry.

The gold-spotted oak borer is native to Arizona but not California. It likely traveled across the desert to rural San Diego County in a load of infested firewood. Researchers have confirmed the presence of the beetle as early as 2000 near the towns of Descanso and Guatay, where nearly every oak tree is infested.

Most of the dead and dying trees are massive, with trunks five or and six feet in diameter, and are 150 to 250 years old. The cost of removing one infested tree next to a home or in a campground can range from $700 to $10,000.

So many oaks have died in the Burnt Rancheria campground on the Cleveland National Forest – a favorite spot for campers who favored the shade of a dense canopy of coast live oaks – that the Forest Service has erected shade structures.

The half-inch-long beetle attacks mature coast live oaks, California black oaks and canyon live oaks -- some of the most important trees used by wildlife for food and cover in California forests and rangelands. The trees seem to have no natural resistance to the gold-spotted oak borer, and no natural enemies of the beetle have been found in the state.

Female borers lay eggs in crevices of oak bark, and the larvae burrow into the cambium layer to feed, irreparably damaging the water- and food-conducting tissues and ultimately killing the tree. Adults then bore out through the bark, leaving a D-shaped hole when they exit.

Unusually high levels of oak mortality were first detected in San Diego's backcountry eight or nine years ago but it took until 2008 to link the problem to the gold-spotted oak borer. The bug was so rare that it didn’t have an accepted common name at the time.

While the borer has been seen in the La Jolla area, the fact that it hasn't spread into other counties is helping to buy time for state and federal scientists looking at pesticides, parasites and other control measures.

"We may still be able to beat thing thing if it's confined to its current location," said Kevin Turner, gold-spotted oak borer program coordinator for the University of California's Agricultural and Natural Resources division.

A network of local, state and federal agencies is working with woodcutters, arborists and consumers to discourage the sale and transportation of infested wood. Wood that is bark-free or that has dried and cured for at least one year is generally safe to transport, Scott said.

Said Turner: “That's biggest tool in our toolbox right now. ... If we can stop the movement of wood, we have good chance in the meantime to come up with other things that will help us gain control."